Re: [PATCH v2 1/5] KVM: x86: Introduce KVM_REQ_RING_SOFT_FULL

[Marc Zyngier]

On Fri, 16 Sep 2022 19:09:52 +0100,
Peter Xu <peterx@xxxxxxxxxx> wrote:
> 
> On Fri, Sep 16, 2022 at 12:51:31PM +0800, Gavin Shan wrote:
> > This adds KVM_REQ_RING_SOFT_FULL, which is raised when the dirty
> > ring of the specific VCPU becomes softly full in kvm_dirty_ring_push().
> > The VCPU is enforced to exit when the request is raised and its
> > dirty ring is softly full on its entrance.
> > 
> > Suggested-by: Marc Zyngier <maz@xxxxxxxxxx>
> > Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
> > ---
> >  arch/x86/kvm/x86.c       | 5 +++--
> >  include/linux/kvm_host.h | 1 +
> >  virt/kvm/dirty_ring.c    | 4 ++++
> >  3 files changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 43a6a7efc6ec..7f368f59f033 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -10265,8 +10265,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> >  	bool req_immediate_exit = false;
> >  
> >  	/* Forbid vmenter if vcpu dirty ring is soft-full */
> > -	if (unlikely(vcpu->kvm->dirty_ring_size &&
> > -		     kvm_dirty_ring_soft_full(&vcpu->dirty_ring))) {
> > +	if (kvm_check_request(KVM_REQ_RING_SOFT_FULL, vcpu) &&
> > +	    kvm_dirty_ring_soft_full(&vcpu->dirty_ring)) {
> > +		kvm_make_request(KVM_REQ_RING_SOFT_FULL, vcpu);
> >  		vcpu->run->exit_reason = KVM_EXIT_DIRTY_RING_FULL;
> >  		trace_kvm_dirty_ring_exit(vcpu);
> >  		r = 0;
> 
> As commented previously - can we use kvm_test_request() instead? because we
> don't want to unconditionally clear the bit.  Instead of making the request
> again, we can clear request only if !full.

I have the feeling that this is a micro-optimisation that won't lead
to much benefit in practice. You already have the cache line, just not
in exclusive mode, and given that this is per-vcpu, you'd only see the
cost if someone else is setting a request to this vcpu while
evaluating the local requests.

And now you need extra barriers...

Also, can we please refrain from changing things without data showing
that this actually is worse than what we had before? The point below
makes me think that this is actually beneficial as is.

> We can also safely move this into the block of below kvm_request_pending()
> as Marc used to suggest.

This, on the other hand, makes sure that we share the cost across all
requests. Requests should be extremely rare anyway (and if they
aren't, you have a whole lot of performance issues on your hands
anyway).

> 
> To explicitly use kvm_clear_request(), we may need to be careful on the
> memory barriers.  I'm wondering whether we should have moved
> smp_mb__after_atomic() into kvm_clear_request() because kvm_clear_request()
> is used outside kvm_check_request() and IIUC all the call sites should
> better have that barrier too to be safe.
>
> Side note: when I read the code around I also see some mis-use of clear
> request where it can be omitted, e.g.:
> 
> 		if (kvm_check_request(KVM_REQ_UNHALT, vcpu)) {
> 			kvm_clear_request(KVM_REQ_UNHALT, vcpu);
> 			vcpu->run->exit_reason = KVM_EXIT_IRQ_WINDOW_OPEN;
> 		}
> 
> Maybe it's a sign of bad naming, so we should renamed kvm_check_request()
> to kvm_test_clear_request() too to show that clearing after that is not
> needed?

Yeah, this kvm_clear_request() is superfluous. But this is rather well
documented, for once, and I don't think we should repaint it based on
a sample of one.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.